Certain cable plants are so lossy that cable modems in remote positions on the network, even when using maximum transmit power, cannot get their transmissions through to the headend modem.
The Cable Modem Termination System (hereafter CMTS) wants to receive bursts from all the remote modems at approximately the same power level regardless of where on the HFC network the modem resides. To accomplish this end, the CMTS receiver in DOCSIS systems for digital data delivery over hybrid fiber coax controls the power of transmissions by each remote modem by a downstream message to each modem. The CMTS controls the power level of transmissions to it by the remote modems so as to keep the received power of each burst at the CMTS within the gain controlled range of the CMTS receiver so as to not saturate its A/D converter and so as to use the full dynamic range of the A/D converter. However, in very lossy systems, some transmitters in the more remote cable modems cannot output enough power to achieve the desired signal power level at the CMTS modem even when their transmit amplifiers are set to maximum transmit power. This causes problems for the CMTS modem, because it does not receive the burst it was expecting from each of these remote modems and some will be outside the range of the AGC and will not be using the full dynamic range of the A/D converter.
Nothing can be done at the remote modem to remedy the problem because the gain of the transmit amplifiers of the remote modems that are having the problem is already set to maximum power. Likewise, nothing can be done at the CMTS modem to remedy the problem since neither the dynamic range of the A/D converter nor the gain controlled range of the CMTS receiver can be changed, so signals with insufficient power will not use the full dynamic range of the A/D converter and will be outside the automatic gain control's ability to control the amplitude of the signal. Nothing can be easily done to lessen the losses in the HFC network.
Something must be done to get more power out of the remote modem without physically putting a more powerful amplifier in it. This problem will be referred to herein as the headroom problem.
Therefore, a need has arisen for an apparatus and method to increase the power of remote modems in lossy systems without actually physically changing the power of the modem transmitter to avoid headroom problems.